One of the least talked-about topics in gardening circles is the quality of water used to water plants. Sadly, not all potable water will grow award-winning plants.

During the drought of 2001, at our Ottawa-area nursery, we had a number of rural customers ask us about their wilting plants, with yellowed foliage and brown leaf margins. They were surprised to learn their irrigation water, like ours, was a prime suspect for their gardening woes. For one three-week period during the 2001 drought, we unwisely resorted to irrigating with our high pH (8.2), somewhat salty well water. Many of our plantings suffered, especially our acid-loving treasures. For ten years we had not seen this detrimental impact of our watering procedure, but past years' acidic rainfalls had been regular enough to keep soil somewhat acidic and flushed of salt. Fortunately, our nursery’s water demands made it necessary to buy water for nursery stock and parched gardens. This was a significant cost (about $225 per 6,000 gallon load) for our business, but with the different, better water, our gardens recovered and made it through a stressful season.

What is 'good' gardening water? Rain water is perfect for just about all plants. It is salt-free and often mildly acidic; just to a plant’s liking – hence the popularity of rain water collection.

What is ‘bad’ water? The answer to this is somewhat like the imprecise definition of a weed as being any plant growing where it is not wanted. It is water which causes poor plant growth. ‘Bad’ water for some uses is however acceptable for others. For example, water that has a high pH and is salt-laden is fine for asparagus, naturally found in salt marshes, but it is definitely not suitable for acid-loving azaleas or bog orchids. Water drawn from an acidic bog would be great for woodland acid-loving plants but would not suit garden vegetables such as beans and cabbages, or baby’s breath and lupins that want a more alkaline growing environment. Most municipally-piped water, private and communal wells and natural water bodies are acceptable irrigation water. Whether you are a rural or urban resident, you should make a point of knowing the pH and salt content of your irrigation water before embarking on ambitious gardening projects that may become dependent upon supplementary water during extended dry periods.

The two separate but not totally unrelated tap water quality concerns for both gardener and gardens are acidity and salt content. Tap water pH is easily measured with an inexpensive pH test kit available from aquarium or garden suppliers. Salts information is available from your municipality for urban folk, or from water softener salesmen or commercial labs for rural gardeners.

Waters with high salt levels should be of most concern to gardeners. This problem is more often found in rural rather than urban areas. ‘Hard’ water with high concentrations of insoluble salts primarily impacts human activities. They cause salt encrustations and soap scums. Water ‘softeners’, designed to solve the homeowners’ plumbing woes, exchange insoluble calcium and magnesium for soluble sodium, which is unhealthy for plants. High salt concentrations around thin-walled root hairs causes water to be drawn out of the roots, by a process called osmosis, to even up the salt concentrations on both side of the root cell walls. Thus, irrigation with salt-rich water creates or exaggerates drought-like stresses on plants.

Rural gardeners are wise to have the salt levels of their irrigation water tested. Ideally, for healthy plant growth, your tap water should contain less than 69 ppm sodium, 71 ppm chloride and 50 ppm bicarbonate. If your tap water contains more than these levels, you should check with a horticulturalist knowledgeable in water quality impacts on plants. In periods of drought gardens require good quality water for survival in a stressful time. Don't kill your plants with salt-laden irrigation water that intensifies root malfunctioning.

Perennials grow and flower best with slightly acidic to slightly alkaline soil conditions between a pH of 5.5 and 7.5. (Water can have a range of acidity levels expressed in pH units from 0-14 with acidic being lower than 7 and alkaline being higher than 7.) Plant root hairs rely on this narrow range of pH conditions to take up moisture and nutrients. Inappropriate irrigation water pH can quickly cause root hair malfunctions resulting in wilting, unhealthy leaves and ultimately plant death.

Two of the more common tap water sources -- municipal and private well water – may present pH problems for some of your garden treasures. Municipal water, depending on your distance from a water purification plant, will likely have neutral to moderately alkaline conditions. Municipalities add lime to treated water to extend the lifetime of their underground pipe system. Enough lime has to be added to the water at the purification plant to maintain an alkaline pH throughout the entire pipe system. The closer your garden is to the water purification plant, the greater the possibility for plant stresses from elevated pH conditions. (It should be noted that human health is not jeopardized by these alkaline conditions. Drinking water which is too acidic, on the other hand, does pose potential health risks - but that is a story for others to tell.)

Different soil types can handle a range of tap water pH levels. Humus-rich garden soils can usually buffer applied waters possessing pH levels outside the ideal range (5.5 - 7.5). Sandy soils have poor ability to buffer pH barrages and plants in these soils are least able to cope with prolonged watering with alkaline water. Clay soils can counteract prolonged exposure to acidic waters, such as acid rain. If your water supply has a pH higher than 7.5, you should consider acidifying any water that will be applied long-term to acid-loving plants. If you don’t grow acid-loving plants and your garden soils are humus-rich, then you may not need to consider water acidification if the only problem with your water is a high pH.

If irrigation is essential for plant survival, you can make alkaline water more acidic. There are a variety of acids for the job. Common vinegar and crystalline citric acid are two of the safest (crystalline citric acid is available at druggists). These have the added benefit of not upsetting soil nutrient conditions. Strong acids such as sulphuric (battery acid) or hydrochloric (muriatic acid used to etch glass or clean masonary) will very quickly burn skin, blind eyes, and put holes in clothing. Other hazardous acids, such as nitric and phosophoric often used in the greenhouse industry, contribute significant nutrients and may cause an unintentional nutrient imbalance while rectifying high pH. If you choose the more hazardous acids, be sure to protect your eyes and clothing with acid-proof goggles and aprons, plus proper gloves for your hands. (Who said gardening was without hazards?)

For small garden areas, consider acidifying one batch of water at a time. To determine the quantity of acid needed to reach your desired water pH, do a trial acidification of a 100 mL water sample. For safety’s sake, never add water to acid because it can react violently and splash onto you! A final pH of 6.0 is adequate for growing most acidic plants in garden soils but you might consider a pH as low as 4.0 for some bog species. Make a succession of small acid additions (1 to 5 mL) to the100 millilitres of water and test for pH after each addition until you arrive at your desired pH. Then multiply the quantity of acid that you used in your 100 mL trial by 10 for preparing each litre of water of your target pH. From then on, just add the calculated amount of acid to every liter in your watering can, and double-check the pH once in a while for peace of mind. For the consummate gardener, routine watering of acid-loving plants with acidic water will keep these plants in top growth.

Home gardeners can acidify constantly flowing tap water using an inexpensive tap add-on called a Hozon Proportioner. The brass Hozon, available at gardening suppliers, is a simple device that has a small diameter rubber tube that sucks solutions of acids or fertilisers into your watering hose. Check results with a pH test kit. Instructions are supplied with the Hozon for preparation of a concentrated acid solution that is diluted in the irrigation process.

Remember, a plant’s root's environment is important to its ability to take water and nutrients from the soil. The gardener’s chore is to maintain appropriate soil pH and salts levels while irrigating in times of low rainfall. Be prepared. Determine your water quality so that you can water with confidence that you are doing your plants a favour.